1. Field of the Invention
The invention relates to a frequency-control circuit, and more particularly to a frequency-control circuit applied in a signal generation device for controlling a frequency of an output signal of the signal generation device.
2. Description of the Related Art
In general, a PWM-type voltage converter comprises an operation amplifier circuit and a comparator (acting as an oscillator) in a feedback path. The ripple frequency of the output signal of the PWN-type voltage converter is determined by the frequency of the saw-tooth signal received by the comparator. However, the operation amplifier circuit comprises many capacitors with large capacitance values and resistors with large resistance values which occupy a large area, and thus the whole area of the PWM-type buck converter is large. Due to the large RC compensation, the transient response of the PWM-type buck converter is slow. Moreover, for PWM-type voltage converters with wide supply (e.g., 5-25 V) and output (e.g., 1-5 V) voltage ranges, it is almost impossible for the ripple frequency of the saw-tooth signal to be eligible for the entire voltage range, and therefore it is difficult to design the operation amplifier circuit for high stability. Thus, a hysteretic voltage converter is provided. In a hysteretic voltage converter, a hysteretic comparator replaces the operation amplifier circuit of the PWN-type buck converter, and the oscillator for the carrier waveform is not needed. This greatly reduces the design and layout area comparing with the PWM type. However, the frequency of the output signal of the hysteretic voltage converter is determined by the hysteresis level. Thus, it is difficult to achieve a fixed constant frequency for the output signal.